The invention relates to a switched reluctance (xe2x80x9cSRxe2x80x9d) motor. More particularly, the invention relates to a method and apparatus for aligning the rotor of an SR motor.
SR motors have multiple poles on both the stator and the rotor. There are windings or coils wound on the stator poles and each pair of windings on diametrically opposite stator poles are connected in series to form an electrically independent phase of the SR motor. There are no windings or magnets on the rotor. However, the rotor is made of a magnetically permeable material such as, for example, a ferrous alloy.
In order to start and run an SR motor, it is necessary to determine the position of the rotor with respect to the stator. The position of the rotor with respect to the stator establishes which phase of the stator is energized or commutated first. If the position of the rotor is not correctly determined, commutation of one of the stator phases may result in inefficient or improper operation of the motor.
Many conventional sensors for determining rotor position exist. Some conventional sensors utilize an encoder. When using an encoder without a home pulse, the homing of the encoder must be done before the SR motor is run. To accomplish homing, the rotor is moved into a desired position and the counter for the encoder is set to a value equivalent to the desired position. Generally, the most accurate position the rotor can be moved to for homing is an aligned position, i.e., when a rotor pole and a stator pole are aligned. The rotor can also be moved to any known position for homing, the most common known position is halfway between two stator poles.
Moving a rotor pole into a desired position is often accomplished by energizing a single phase of the motor. When a single phase of the motor is energized, current flows through a winding on a stator pole of the energized phase and produces a magnetic field. The magnetic field attracts a rotor pole that moves toward the stator pole of the phase that has been energized. When the rotor pole moves into alignment with the stator pole there is often enough inertia to keep the rotor pole moving right past the aligned position. A pendulum-type motion results. The duration of the oscillatory movement depends upon the ratio of inertia to friction, or the damping ratio. Oscillatory movement can continue upwards of ten to fifteen seconds in some applications where SR motors are used (e.g., some types of blowers). In devices with higher damping ratios the duration of the oscillatory movement may be much shorter. Once the oscillatory movement has ceased, the rotor pole will be aligned with the stator pole (i.e., aligned position) and the encoder can be homed.
Another way of moving a rotor pole into a desired position is accomplished by simultaneously energizing two phases of the motor independent of each other. This method is similar to the method discussed above of energizing a single phase of the motor except that it is equivalent to having a two point gravity source and therefore only widens the oscillatory movement. This method typically results in a longer decay time than the single phase energized method. Once the oscillatory movement has ceased, the rotor pole will be aligned halfway between the two energized stator poles (i.e., known position).
Other ways of moving a rotor pole into a desired position that result in oscillatory movement of the rotor are not acceptable for applications where a fast response of the motor is required. The time that the oscillatory motion takes to decay delays homing of the encoder and therefore delays operation of the motor.
Accordingly, the invention provides a way of moving a rotor pole into a desired position with little to no oscillation. The automatic feedback method of the present invention simultaneously energizes two phases of the motor using a combined current method. Instead of controlling each phase current independently, the sum of the phase currents is controlled resulting in the natural damping of the oscillations. The rotor gently moves into a desired position with little to no oscillation. When two phases are energized in this manner, the rotor pole will come to rest halfway between two stator poles corresponding to the two phases that are energized.
An encoder can be homed at a position halfway between two stator poles if that position can be accurately determined. However, a problem that can be encountered when a rotor is aligned halfway between two stator poles is that the measurement of the inductance present in each of the phases may not be accurate enough to home the encoder. This problem is often encountered when high efficiency motors are used. High efficiency motors generally utilize differing degree stator arcs and rotor arcs (for example, a 30 degree stator arc and a 32 degree rotor arc). The overlap resulting from the differing arcs of the stator and the rotor typically do not cause great enough differences in inductance measurements to accurately home the encoder for efficient operation of the motor.
Accordingly, to alleviate this problem when it exists, the invention further provides a way of gently moving the rotor pole from a known position to an aligned position. A time based multiplier is utilized to adjust the duty cycle command of one of the phases, this allows the current to be reduced in that phase until all the current is in one phase only. To prevent the introduction of oscillatory movement, the time based multiplier waits until after the rotor has come to rest half way between two stator poles or has only minimal movement before adjusting the duty cycle The rotor will rotate to the stator pole of the phase with all the current. When the rotor pole and stator pole are aligned they will be in an aligned position. When the rotor pole is in the aligned position the encoder can be accurately homed.